A differential amplifier has a voltage offset caused by variations in characteristics of elements at a time of manufacture. When, for example, an output of a sensor is to be amplified, an offset voltage is added to an output signal of the sensor or subtracted therefrom, so high-precision measurement cannot be performed.
Therefore, according to Patent Document 1, an input offset is cancelled by a circuit structure shown in FIG. 6 to perform processing for removing the influence of the offset on the output signal of the sensor to be amplified.
That is, as shown in FIG. 6, when amplification processing is to be performed, gate electrodes of MOS transistors 101 and 102 included in a first input differential pair are short-circuited. Therefore, a state in which the same input voltage is inputted to a differential amplifying circuit is obtained, with the result that a voltage appearing at an output terminal is inputted to an amplifier 100 for measuring an offset voltage.
The amplifier 100 is an amplifier for inverting and amplifying an inputted voltage. Feedback control is performed so as to cancel an offset added by the MOS transistors 101 and 102 to obtain the offset voltage. The offset voltage is held by a capacitor CINT.
As a result, the offset voltage is negatively fed back to a gate electrode of a MOS transistor 103 included in a second differential pair for controlling output current terminals of the first input differential pair. A predetermined voltage is applied to a gate electrode of a MOS transistor 104 included in the second differential pair. According to the above-mentioned processing, the amplification can be performed by the first input differential pair while the offset voltage is substantially cancelled.
[Patent Document 1] U.S. Pat. No. 4,933,642
Up to now, a 1/f noise generated in a MOS transistor at a low frequency has been known as a noise of the amplifier.
The 1/f noise has a characteristic that it increases as a frequency reduces. Therefore, when the MOS transistor is used to amplify a sensor output, the sensor output is significantly influenced by the 1/f noise because a large number of sensor outputs which are to be amplified are relatively low-frequency signals. Thus, even when the voltage offset is removed, the amplification processing cannot be performed on the sensor output at high precision.
A 1/f noise of each of the MOS transistors 103 and 104 is added to a 1/f noise of the amplifier 100, so a total 1/f noise becomes larger than that of a normal noise.
Up to now, a chopper amplifier is used as described in Patent Document 1. However, there is a problem about the offset as described above, and that the circuit structure for removing the offset becomes complicated. Therefore, as to an amplifier used for a small-size and low-power consumption product, there are disadvantages in which a circuit scale becomes larger, a cost rises, and power consumption increases.
The present invention has been made in view of the above-mentioned circumstances. Therefore, an object of the present invention is to provide a switched capacitor amplifier capable of removing the influence of an offset voltage and a 1/f noise by using a simple circuit, and a method of operating the switched capacitor amplifier.